1. Field of Invention
This invention relates to light emitting devices such as light emitting diodes (LEDs) and, in particular, to techniques for mounting LEDs for packaging and for removing the growth substrate of the LEDs.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Sapphire is often used as the growth substrate due to its wide commercial availability and relative ease of use. The stack grown on the growth substrate typically includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region.
Devices formed on conductive substrates may have the p- and n-contacts formed on opposite sides of the device. Often, III-nitride devices are fabricated on insulating or poorly conducting substrates with both contacts on the same side of the device. Such devices are often mounted so light is extracted through the growth substrate (known as a flip chip device).
III-nitride LED structures are often grown on sapphire substrates due to sapphire's high temperature stability and relative ease of production. In a flip chip device, the use of a sapphire substrate may lead to poor extraction efficiency due to the large difference in index of refraction at the interface between the semiconductor layers and the substrate. When light is incident on an interface between two materials, the difference in index of refraction determines how much light is totally internally reflected at that interface, and how much light is transmitted through it. The larger the difference in index of refraction, the more light is reflected. The refractive index of sapphire (1.8) is low compared to the refractive index of the III-nitride device layers (2.4) grown on the sapphire. Thus, a large portion of the light generated in the III-nitride device layers is reflected when it reaches the interface between the semiconductor layers and a sapphire substrate. To contribute to the output the reflected light must be scattered into the escape cone or reflect within the structure until it reaches the edge of the device where with small probability it may emerge as sidelight. The optical losses at the contacts and within the III-nitride device layers, e.g. free carrier absorption, significantly attenuate this trapped light before it escapes. Incorporation of an effective optical scatterer within the structure, preferably within the high index layer of the structure, increases the probability of the light being scattered into the escape cone before it is absorbed within the structure.
It is sometimes desirable to remove the growth substrate, for example, to improve the optical properties of the LED, or to gain electrical access to the LED layers. In the case of a sapphire substrate, removal may be by means of laser dissociation of GaN at the GaN/sapphire interface. In the case of Si or GaAs substrates, more conventional selective wet etches may be utilized to remove the substrate. Laser dissociation in particular generates shock waves in the semiconductor layers which can damage the semiconductor or metallization layers, potentially degrading the performance of the device.
Needed in the art are device designs and methods that facilitate substrate removal without damaging the semiconductor, that are compatible with available manufacturing techniques, and that enable advanced device designs.